Linear transmitters are used in a wide variety of application, including mobile communications. In particular, linear transmitters are used in digital communications because of the excellent spectrum efficiency they offer. The spectral efficiency of linear modulation is the reason why it is fast becoming the preferred wireless data transmission modulation in many fields.
A typical linear transmitter includes a baseband signal generator that produces a baseband signal output in response to a digital input. The digital input is mapped to a constellation corresponding to output voltage level and phase. An example of such a modulation is known as quadrature amplitude modulation (QAM), which is a popular form of modulation. The baseband signal is fed to an up mixer to create a radio frequency signal, which is then filtered and fed to a power amplifier for transmission over the air.
In some applications increased linearity was desired, and Cartesian feedback was found to significantly enhance linearity. Cartesian feedback comprises sampling the output of the power amplifier, down mixing it at the same frequency the up mixer uses but at a phase delay, then feeding the down mixed signal back to a combiner or summing node and subtracted from the output of the baseband signal generator. Instead of feeding the baseband signal directly to the up mixer, the difference of the baseband signal and the down mixed signal is amplified and fed to the up mixer. Cartesian feedback is discussed in, for example, U.S. Pat. No. 5,420,536 to Faulkner, and U.S. Pat. No. 5,933,767 to Leizerovich et al.
In mobile applications the gain of the transmitter's power amplifier becomes an important consideration because increased gain results in excessive out of band noise which may interfere with nearby receiver, while decreased gain may result in insufficient transmit power. Nearby receivers may be affected by out of band noise because the out of band noise in the transmit signal may intrude into the receive frequency band of a nearby receiver. This is particularly true in time and frequency divisioned communication systems where transmit and receive frequency pairs are used. In mobile communication devices, which experience significant changes in temperature over relatively short periods of time, the gain of the power amplifier can change during transmission. If the gain of the power amplifier goes up, the increase in gain is automatically compensated by use of a Cartesian feedback loop which reduces the level of the signal fed to the upmixer. This method proved adequate when used with significant filtering. The filtering is necessary because of the out of band content added by baseband amplifiers, upmixer, and other components in the transmit path. Typically surface acoustic wave (SAW) filters are used to reduce out of band spectral content. SAW filters tend to use a significant surface area, and therefore require significant space. However, because mobile communication devices are sold in a cost and size sensitive market, it is desirable to eliminate these filters. This is especially true if the mobile communication device is designed to operate in more than one frequency band because a different filter will be required for each band, and radio frequency switches will be needed to switch the signals between the appropriate filters.
It has been found that filters can be avoided with the use of low noise mixers. Under nominal conditions the out of band spectral content introduced by the low noise mixers is low enough to be acceptable. However, if the gain of the power amplifier increases due to, for example, temperature changes of the power amplifier's active device, the out of band content gets amplified as well. Were it not for feedback, the increase in gain would cause both the in band and out of band signal content to become amplified. Of course, feedback prevents the in band content from being over-amplified, but because the source of the out of band noise is not significantly affected by feedback, it is amplified more. Without filters the out of band content can be amplified to an unacceptable level and interfere with nearby receivers operating on nearby frequencies. The routine way of compensating for gain increases of lowering the input signal is ineffective because the out of band content is added by the mixer, and tends to be substantially constant. Therefore there is a need for a way to compensate for thermal effects on the power amplifier, in a low noise filterless transmitter.